The psychopharmacology and neurochemistry of hallucinogenic drugs are investigated in relation to the similarity or dissimilarity of their behavioral effects in rats. Dose-response studies with prototypical indoleamine and phenyethylamine hallucinogens, LSD, tetrahydrocannibinol, and phencyclidine are conducted to determine the influence of each drug on the rat startle response to tactile or acoustic stimuli. Their sites of action in the dopaminergic, serotonergic, and noradrenergic systems of discrete brain regions are explored using pharmaceutical manipulations with antagonists and selective neurotoxic lesioning techniques.